Immunotherapy (allergen injection therapy) is in common use in the treatment of childhood allergic asthma. Most of the existing clinical studies purporting to demonstrate the efficacy of this form of treatment are experimentally flawed, or do not address the clinically relevant issue of multiple allergen immunotherapy for perennial asthmatic symptoms. The principal objective of this research is therefore to evaluate in a prospective, randomized, double-blind clinical trial the effectiveness of optimal allergen immunotherapy on the severity and course of perennial asthma in atopic children. A group of allergic asthmatic children, age 5 to 12, will be characterized according to disease severity and medication requirements. Following a 2 to 3 month period of medication stabilization, each will be randomized to receive placebo or active immunotherapy with aqueous extracts of pertinent environmental allergens. Treatment will be given bi-weekly for 3-5 years. One team of investigators will remain blinded to supervise clinical care; a second unblinded team will administer the immunotherapy, and monitor progress in terms of immunologic and clinical outcomes. Disease severity will be assessed primarily by the type and quantity of medication required to achieve acceptable symptom scores and pulmonary function tests. Medications will be adjusted frequently to maintain symptoms and pulmonary function within targetted limits. Three additional research objectives will be pursued in this same set of well-characterized asthmatic children. First the natural history of airway hyperreactivity to non-specific stimuli will be evaluate using periodic metacholine inhalation tests. The effect of immunotherapy on airway hyperreactivity will be determined by comparing changes in active treatment and placebo groups. Second, the effect of long-term immunotherapy on target cell sensitivity in children as manifested in basophil histamine release and in wheal-and-flare responses of skin mast cells will be studied by periodic assessment of these functions in the two treatment groups. Third, benefit of environmental analysis of the home will be studied. Clinically significant household allergens including animal dander, dust mites (Dermatophagoides species), and common mold spores will be assessed by home inspections, the use of mold culture plates, and antigenic analyses of aqueous extracts of house dust samples. The effectiveness of cleaning procedures of aqueous extracts of house dust samples. The effectiveness of cleaning procedures on the quantity and distribution of household allergens will also be determined.